From Wiki: The botfly is a family of Oestroidea. It is one of several families of hairy flies whose larvae live as parasites within the bodies of mammals. Dermatobia hominis, or human botfly, is the only species of botfly known to use humans as the host to its larvae.

@James O'Dwyer — but the botfly is a parasite. Yes, it uses mammals for its larvae to grow in... but the process doesn't kill its host.

Parasitoids do the same thing, but kill the host. Take the jewel wasp, for instance.

emale wasps of this species sting a roach (specificially a Periplaneta americana, Periplaneta australasiae or Nauphoeta rhombifolia[1]) twice, delivering venom. A 2003 study[2] proved using radioactive labeling that the wasp stings precisely into specific ganglia of the roach. She delivers an initial sting to a thoracic ganglion and injects venom to mildly and reversibly paralyze the front legs of the insect. This facilitates the second venomous sting at a carefully chosen spot in the roach's head ganglia (brain), in the section that controls the escape reflex. As a result of this sting, the roach will first groom extensively, and then become sluggish and fail to show normal escape responses.[3] In 2007 it was reported that the venom of the wasp blocks receptors for the neurotransmitter octopamine.[4]

The wasp proceeds to chew off half of each of the roach's antennae.[1] Researchers believe that the Wasp chews off the antenna to replenish fluids or possibly to regulate the amount of venom because too much could kill and too little would let the victim recover before the larva has grown. The wasp, which is too small to carry the roach, then leads the victim to the wasp's burrow, by pulling one of the roach's antennae in a manner similar to a leash. Once they reach the burrow, the wasp lays a white egg, about 2 mm long, on the roach's abdomen. It then exits and proceeds to fill in the burrow entrance with pebbles, more to keep other predators out than to keep the roach in.

With its escape reflex disabled, the stung roach will simply rest in the burrow as the wasp's egg hatches after about three days. The hatched larva lives and feeds for 4–5 days on the roach, then chews its way into its abdomen and proceeds to live as an endoparasitoid. Over a period of eight days, the wasp larva consumes the roach's internal organs in an order which guarantees that the roach will stay alive, at least until the larva enters the pupal stage and forms a cocoon inside the roach's body. Eventually the fully-grown wasp emerges from the roach's body to begin its adult life. Development is faster in the warm season.

My Dad has something similar to that. He had come back from Ethiopia with some odd bites that went all puss-filled, got board of them so took a knife to them only to find grubs in side. He pulled 8 out of his knee and 4 from his arm, infront of us, after dinner.

@Ctrlaltdan - that sounds like a tumbu fly. Tumbu flies lay their eggs on your washing while it's hanging on the clothesline, then the grubs hatch and burrow into your skin when you put on an item of infected clothing.

Still, I bring to your attention a creature you may NOT want to swim near if it is horny, desperate, and has already lowered its standards: the Greater Hooked Squid (Moroteuthis ingens)

If this was a movie, I would call it 'Attack of the rapist sperm' and get Noburo Iguchi to direct.

"With ... the Moroteuthis ingens, the spermatophores are introduced in a more peaceful way. ‘With this species the spermatophores penetrate the skin independently. They probably do that with the help of an enzyme-like substance that dissolves tissue.’ Hoving is the first to be able to prove that these sperm packets are able to penetrate the skin under their own steam. He discovered this when he experimentally placed spermatophores on the skin of just-caught individuals. His results are supported by an incident in Japan, where someone had to have an operation after eating squid to remove a spermatophore that lodged in his throat.

Many animals employ mimicry. Many cephalopod species have chromatophores, cells that can change color.

One species of cephalopod, the mimic squid, uses its color-changing abilities and its felxible body to assist its mimicry.

But it doesn't just mimic one thing, the mimic squid can imitate a whole range of different fish and even sea snakes (by hiding it's body and six legs in a hole and putting the remaining two legs in single line.

Furthermore, it chooses what to imitate based on the species it's trying to fool.

For example, it only imitates the sea snake in the presence of the damselfish which is prayed on by sea snakes.

@RenThing and racingpenguins: You can take heart in the fact that they've found Great Whites really don't like eating humans — we're too bony, and not blubbery enough. Scientists think that the vast majority of Great White 'attacks' on humans are them "test biting" to see exactly what we are. But still — EEK!

@Kosmopolit — I think you mean the mimic octopus. Here's a pretty good video of it. (And what the hell is that first creature??)

@Ren - that shark story is true. The shark that was bitten (almost) in two was caught on a drumline on a popular tourist beach in Queensland, and the bite marks were just as large as they appear in the photo.